Secured fiber link system

ABSTRACT

A fiber link system, method and device for masking signals on a fiber link system. The system includes sending a desired sequence of information in the form of a true signal that is typically intended to be transferred between legitimate users at both ends of a link. Sending chaff signals, or subterfuge signals, alongside the true signal to mask such legitimate signals in the fiber cable from intruders tapping into the fiber cable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/435,619, filed Feb. 17, 2017, now allowed, which claims the benefitof U.S. Provisional Application No. 62/296,897 filed on Feb. 18, 2016and U.S. Provisional Application No. 62/301,892 filed on Mar. 1, 2016,the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to optical fiber cables, andmore particularly to securing information traversing optical fibercables.

BACKGROUND

Intruders can tap into optical fiber transmission lines and stealinformation by bending the fiber to enable reading and interpreting thesignal energy escaping from that fiber. While there are other methods oftapping information out of optical fibers, taps based on fiber bendingare easy to implement, effective, and can be hard to detect. Tappingvaluable data transmitted over the world wide optical fiberinfrastructure is a threat to every major industry and governmentorganization and, in particular, larger organizations utilizing multiplefacilities. While these organizations may be able to secure opticalfiber cables within their own facilities, they generally have much lesscontrol over the optical fiber cable links between those facilities.

The vulnerabilities of optical fibers to tapping and, in particular,tapping by bending, leaves many organizations susceptible to data theft.Such data theft could lead to leaking of confidential information,thereby causing harm to the entity transporting data over opticalfibers. In the business context, leaking of information such asmarketing strategies or developmental technologies may ultimately resultin damage to profits. In the governmental context, leaking ofinformation related to national security may endanger the lives ofcitizens. Existing solutions face challenges in detecting and preventingtapping of optical fibers.

It would be advantageous to provide a solution that would overcome thedeficiencies of the prior art.

SUMMARY

A summary of several example embodiments of the disclosure follows. Thissummary is provided for the convenience of the reader to provide a basicunderstanding of such embodiments and does not wholly define the breadthof the disclosure. This summary is not an extensive overview of allcontemplated embodiments, and is intended to neither identify key orcritical elements of all embodiments nor to delineate the scope of anyor all aspects. Its sole purpose is to present some concepts of one ormore embodiments in a simplified form as a prelude to the more detaileddescription that is presented later. For convenience, the term “someembodiments” may be used herein to refer to a single embodiment ormultiple embodiments of the disclosure.

Some embodiments disclosed herein include a system for securing fiberlinks including: a spatially multiplexing optical fiber, a transmitspatial multiplexer configured to couple a plurality of optical signalsinto a plurality of spatial paths of the spatially multiplexing opticalfiber, at least one optically modulated transmit source, at least oneoptically modulated chaff source, and a receive spatial multiplexerconfigured to extract at least one more spatial path from the spatiallymultiplexing fiber.

Some embodiments disclosed herein also include a fiber terminalincluding: an optical coupler for interfacing between at least oneindividual true signal channel and at least one chaff signal channel andan optical cable link, a chaff clock synchronizer configured for clockrecovery and/or synchronization, a chaff signal generator configured todrive the chaff channels in synch with the true signal channels, andtransceivers configured to convert between electrical input/outputsignals and optical signals that are transmitted over the optical cablelink.

Some embodiments disclosed herein also include a method for securingfiber links reciting: coupling a plurality of optical signals into aplurality of spatial paths of a spatially multiplexing optical fiber,wherein there are at least one optically modulated transmit sourcescomprising real data and at least one optically modulated chaff source.The method may further include extracting by a receive spatialmultiplexer at least one spatial path from the spatially multiplexingfiber.

Embodiments may be further defined where the data streams are at leastone of generated at a chaff transmitter in real time, pre-recorded andstored at the chaff transmitter, or generated at the chaff transmitterusing a quantum random number generator.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter disclosed herein is particularly pointed out anddistinctly claimed in the claims at the conclusion of the specification.The foregoing and other objects, features, and advantages of thedisclosed embodiments will be apparent from the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of a secured fiber link system.

FIG. 2 is a block diagram of a seven-channel multicore fiber.

FIG. 3 is a cross-sectional view of a fiber showing a refractive indexprofile.

DETAILED DESCRIPTION

It is important to note that the embodiments disclosed herein are onlyexamples of the many advantageous uses of the innovative teachingsherein. In general, statements made in the specification of the presentapplication do not necessarily limit any of the various claimedembodiments. Moreover, some statements may apply to some inventivefeatures but not to others. In general, unless otherwise indicated,singular elements may be in plural and vice versa with no loss ofgenerality. In the drawings, like numerals refer to like parts throughseveral views.

Various disclosed embodiments include a secured or untappable fiber linksystem that can operate in concert with other measures for protecting orsecuring transmitted data such as data encryption, patrolling of datalines by guards, intrusion detection monitor sensors, and hardening ofdata lines by encasing them in concrete or steel conduits.

In an embodiment, the secured fiber link system is configured to allowtransparent transmission of data to the intended user whilesimultaneously making the signal opaque or uninterpretable to anintruder anywhere along the entire link by overwhelming the realinformation signal with interfering signal energy. In one embodiment,the secured fiber link system is compatible with any contemporary datarates, formats or telecommunications protocols. As such, the disclosedsystem may be agnostic to new equipment and protocols that will bedeveloped in the future.

According to the disclosed embodiments, the secured fiber link systemenables physical security of in-flight data propagating over a securedlink by preventing the acquisition of the transmitted real data by anintruder, without corruption to the real data. This is achieved by chaffsignals generated at the terminal equipment and transmitted on theoptical fiber. The chaff signals may be generated and transmittedwhether or not the real data is encrypted. The system is configured toensure that the intended recipient at the other end of the link receivesthe real data without receiving the chaff signals or corrupting the realdata signal. Encrypting the data stream takes up transmission bandwidthand can introduce latency in the data transmission and even an encryptedsignal can be tapped by an intruder. But the secured fiber link systemprevents the intruder from acquiring the real data in an unaltered form.

In an embodiment, the secured fiber link system can be used inconjunction with encryption to further secure data transmitted betweenterminals or can reduce the need for encryption which would in-turn freeup considerable transmission bandwidth in the link.

The secured fiber link system sends a desired sequence of information inthe form of a first “legitimate” or “true” signal (or set of signals),which is the real signal that is typically intended to be transferredbetween legitimate users at both ends of a link. Chaff signals, orsubterfuge signals, are signals sent alongside the true signal toprevent intruders from misappropriating information being transmittedover a fiber cable by tapping such legitimate signals from a fibercable.

According to one embodiment, the “true” signals look like chaff signalsto an intruder. This is achieved by applying a synchronization or clockrecovery technique. In an embodiment, a synchronization technique,discussed in detail below, is utilized to align a chaff signal generatorwith a signal channel at the transmitter.

An intruder may be any entity that is not the intended recipient of thelegitimate signal that attempts to misappropriate information beingtransmitted over a fiber cable by tapping the cable. The intrudertypically attempts to extract data from a fiber cable via mechanicalmethods such as, e.g., bending, thereby creating scattered or evanescentlight energy that may be captured from either the core or cladding of afiber. Tapping may include, but is not limited to, altering a fibercable by exerting force on the cable (e.g., by bending the cable) suchthat the energy of the fibers within the cable is diverted and capturedby the intruder.

FIG. 1 shows an exemplary and non-limiting block diagram of a securedfiber link system 100 according to an embodiment. The system 100includes transceiver terminals 110-1 and 110-2 connected via opticalfiber cables 150-1 and 150-2. Each terminal 110-1 or 110-2 includestransmitters 121-1 and 121-2 or 122-1 and 122-2, a receiver 130-1 or130-2, and a chaff generator 140-1 or 140-2, respectively. Merely by wayof example and without limitation on the disclosed embodiments, FIG. 1will be discussed herein below with respect to transmitting signals fromthe terminal 110-1 and receiving the signals at the terminal 110-2.Signals may be equally transmitted from the terminal 110-2 and receivedat the terminal 110-1 without departing from the disclosed embodiments.

The terminal 110-1 transmits and receives signals via the optical fibercables 150-1 and 150-2, respectively. One or more chaff (i.e.,interfering) signals may be generated within the terminal 110-1 andtransmitted via the optical fiber cable 150-1 along with the true signalsent by the transmitter 121-1. The chaff signals may be generated withinthe chaff generator 140-1 or 140-2 and transmitted via the transmitter121-2 or 121-1. The properties of the chaff signals are “matched” to thetrue signal at one of the receivers 130-1 or 130-2. The chaff signalsare indistinguishable, in terms of data rate, wavelength spectrum, dataformat and protocol, from the true signal except for the difference ininformation content.

Tapping the optical fiber cable 150-1 anywhere along the link willresult in capturing energy from both the real signal core(s) and fromchannels carrying chaff signal(s), whereas the intended recipient of thesignal information content (at the other end of the link) is not exposedto the chaff signals received at the terminal 110-2. As a result, anintruder tapping into the optical fiber cable 150-1 anywhere along thelink between the terminals 110-1 and 110-2 detects a mixture of thechaff and true signals which cannot be distinguished from each other,thereby protecting the data of the true signal from the intruder. At thereceivers 130-1 and 130-2, the intended recipient will detect only thetrue signal without interference from the chaff signal.

It should be noted that only two terminals 110 and two optical fibers150 are shown in FIG. 1 merely for simplicity purposes and withoutlimitation on the disclosed embodiments. Additional terminals and/oroptical fibers may be utilized without departing from the scope of thedisclosure.

FIG. 2 is an exemplary and non-limiting schematic diagram of a n-channelmulticore fiber terminal 200 according to an embodiment. The terminal200 includes a coupler 210, a chaff clock synchronizer 220, a chaffsignal generator 230, and transceivers 240-1 through 240-n (hereinafterreferred to individually as a transceiver (Tx/Rx) 240 and collectivelyas transceivers 240, merely for simplicity purposes, ‘n’ is an integergreater than 1). In an embodiment, the n-channel multicore fiberterminal 200 is a 7-channel fiber terminal including 7 transceivers 240.

The coupler 210 may be a bi-directional optical coupler for interfacingbetween individual true and chaff signal channels and the optical cablelink. The coupler 210 is a structure to couple a true signal and atleast one or as many as six chaff signals, in this example, into theindividual channels of the multicore fiber. The coupler 210 may be, butis not limited to, a lensed fiber based coupler, a tapered glass fibercoupler, a polymer based coupler, and a free space bulk optics coupler.Each signal is transmitted via one of the transceivers 240. Thetransceivers 240 convert between electrical input/output signals andoptical signals that are transmitted over the optical cable link. Thecoupler 210 and the transceivers 240 may be a transmit spatialmultiplexer configured to couple a plurality of optical signals into aplurality of spatial paths of a spatially multiplexing optical fiberfrom one or more optically modulated transmit sources and one or moreoptically modulated chaff sources. On the receive side a receive spatialmultiplexer may be configured to extract one or more spatial paths fromthe spatially multiplexing fiber.

In an embodiment, the transceivers 240 operate at or near the sameoptical wavelength for all channels. In the non-limiting embodimentshown in FIG. 2, the transceiver 240-4 is associated with a channel fortransmitting the true signal, and the transceivers 240-1 through 240-3and 240-5 through 240-n are associated with channels for transmittingchaff signals. In an embodiment, at the junction 201 which is the inputto the transceivers, the signal channel is separated from the chaffsignals.

The chaff signal generator 230 is configured to drive the chaff channeltransceivers in synch with the true signal channel. The chaff-clocksynchronizer 220 may be utilized for clock recovery and/orsynchronization. In an embodiment, the clock recovery and/orsynchronization may be performed at a transmitter such that broadcastchaff signals can synch-up with the signal channel(s). The same basicmethod can be used for commonly used data rates such as, but not limitedto, 1 Gbit/s, 10 Gbit/s, 100 Gbit/s, and the like, and common forms ofdigital signal formats such as, but not limited to, different forms ofon-off keying (OOK), phase shift keying (PSK) and frequency shift keying(FSK).

The waveforms can also follow standard SONET or SDH packet format. Thesynchronizer 220 is configured to synchronize the chaff signals toappear identical to the true signal except for the information content.In an embodiment, the signal levels of the chaff channel or channels aresufficient to cause a significant degradation of the BER, SNR, andquality of the eye-diagram of the signal received by the intruderrelative to that observed by the intended recipient at the other end ofthe link.

According to certain embodiments, the chaff clock synchronizer 220 and achaff signal generator 230 can be implemented as a chipset, amicrocontroller, a field programmable gate array (FPGA), a programmablelogic device (PLD), an application specific integrated circuits (ASIC)or any other type hardware components. In another embodiment, the chaffclock synchronizer 220 and a chaff signal generator 230 can beintegrated in an optical transceiver system.

In another embodiment, the transmission (optical) wavelengths of thechaff signal transceivers should be close to that of the signal channeltransceiver based on the knowledge of one ordinarily skilled in the art.This is one more potential discriminator between the chaff and signalchannels that an intruder may exploit to extract signal information. Inan embodiment, the small delta wavelength ensures that the intrudercannot separate the signals and chaff.

The secured fiber link system may provide several advantages such as,but not limited to, protection along the entire length of the linkwithout the need for expensive guards or encasements making installationand security maintenance less expensive (especially on a long link), andincrease in bandwidth available for transmitting data due to reduceddependence on data encryption for security.

The chaff signal channels received at the terminal equipment can,however, be used to facilitate intrusion detection and in fact, can beused in conjunction with several existing intrusion detection methods.Thus, a secured fiber link system including the terminal 200 does notimpede other security technologies used to prevent intrusion in opticalfiber cables.

In an embodiment, the chaff signals may be similar to the true signal insignal protocol/format, (sample) timing, and/or optical wavelength suchthat both the chaff signals and the true signal will pass through anydiscriminator, used by an intruder, based on any or all of those signalfeatures. Therefore, given that the chaff signal(s) have sufficientenergy, the intruder will be swamped by the chaff information which theycannot separate from the true signal information. Given that the chaffsignals are strong enough compared to the true signal along the entirelength of the cable, the intrusion defense will be strong everywherealong the cable without the need for additional protection such as cablearmament or posting sentries. In another embodiment, the signal andchaff channels may also be utilized for intruder monitoring.

There are a number of modalities within which the secured fiber linksystem can be effective against fiber tapping. In the first modality,the chaff signals are designed to appear in the same format andsynchronized timing (clocking) and wavelength that makes them virtuallyindistinguishable from the true signal(s), except for their informationcontent. This means that, except for information content, the chaffsignals are virtually indistinguishable both spectrally and temporallyfrom the true signals being transmitted. Therefore, the intruder cannotdetermine that there are signals different from the true signal withoutprior knowledge about the information content of the true signal.

In a different modality, the chaff signals can be made overwhelminglystrong so that the level of tapped optical energy exceeds the dynamicrange of the detector in the intruder's tapping equipment. In this case,there may not be any need to structure the format of the chaff signalsas their total power will overwhelm the tapping equipment.

It should be noted that FIG. 2 is described herein above with respect toone true signal and six chaff signals merely for simplicity purposes andwithout limitation on the disclosed embodiments. More or less chaffsignals and additional true signals may be utilized without departingfrom the scope of the disclosure.

According to various disclosed embodiments, a secured fiber link systemmay include a transmission medium and chaff or subterfuge signaling, andmay be compatible with fiber networks. The transmission medium may be anoptical fiber cable in which the transfer of information from a source(sender) is nearly transparent, having a high signal to noise ratio(SNR) and low bit-error-rate (BER), to intended user(s) but is virtuallyopaque (having low SNR and high BER) to an intruder attempting to tapthe cable anywhere along the line.

The chaff or subterfuge signaling enables making the link opaque tointruders. Implemented at the terminal equipment, “synchronizing” theinterference data with the desired true signal makes it virtuallyimpossible for the intruders to separate the true signal from theinterfering signals without prior knowledge of the true signal. Thechaff signals generated at the terminal may be compatible with anycurrent or future transmission formats and protocol. The informationcontent of the chaff signals should be totally uncorrelated with thereal data. This can be achieved by, but not limited to, a random datagenerator or by using a pre-recorded set of data to be transmitted aschaff. Additionally, the secured cable system may be compatible withexisting fiber cable infrastructures, thereby allowing seamlessintegration of the secured cable system into existing infrastructures.Thus, the plurality of optically modulated chaff sources may containdata streams where the data streams may be any combination of one ormore of generated at the chaff transmitter in real time, arepre-recorded and stored at the chaff transmitter or are generated at thechaff transmitter using a quantum random number generator.

According to various disclosed embodiments, the transmission medium mayinclude, but is not limited to, any fiber in which multiple, independentinformation bearing signals can propagate simultaneously. Examplesinclude, but are not limited to, multicore optical fibers whereindividual signals can propagate in a set of parallel cores and anyfiber, including multicore fibers, few mode fibers and multimode fibers,where spatial division multiplexing technology is employed. While theprinciples of the disclosed embodiments may apply to all of these typesof fibers, for the sake of simplicity, the disclosure is discussed withrespect to independent signals each propagating in a core of a multicorefiber. It is further assumed that there are at least one or morededicated channels used to transmit signals in the multicore fiber.Furthermore, it is assumed that there is at least one chaff channel andat least one signal channel propagating in the multicore fiber.

The anti-tapping system being utilized according to various disclosedembodiments, results in protection regardless of how an intruder tapsthe fiber to attempt signal pilfering. For the current secured system,it is assumed that the chaff and true signal channels can each becoupled selectively into the chaff and true signal channels of the fiberat the transmitter end and selectively coupled out at the other end(receiver) of the link with an appropriate coupler. Such couplersinclude lensed fiber based couplers, tapered glass fiber couplers,polymer based couplers, and free space bulk optics couplers.

In an example embodiment, the optical fiber medium may have thefollowing properties that both maximize the effectiveness of theanti-tapping capability of the system and do not inhibit the legitimateinformation transfer between system users: out-coupling efficiency ofthe true and chaff channels via bending, attenuation of the chaff andtrue signal channels, and cross-talk between the chaff and true signalchannels.

With respect to out-coupling, the fiber medium typically has radialsymmetry with respect to its refractive index cross section. This wouldimply that the out-coupling of light is the same regardless of which waythe intruder bends the fiber. Define the symbol p (dB) as the fractionof energy coupled out of the fiber by a tapping mechanism. Then, thefollowing three conditions are possible, with ρ_(chaff) being the energycoupled from the chaff signal(s) and ρ_(signal) being the energy coupledfrom the true signal(s).

If ρ_(chaff)>ρ_(signal), then the out-coupled chaff signal can overwhelmthe out-coupled real signal, however, it may be possible to strip outthe chaff signal upstream of the receiver and then tap the real signaldownstream with minimal interference from chaff.

If ρ_(chaff)<ρ_(signal), then it is possible for the signal energytapped out to overwhelm the chaff signal tapped out of the fiber.

If ρ_(chaff)=ρ_(signal), the fiber can be made secured and untappablealong its entire length.

In an example embodiment, the first condition is held so that at allpoints along the fiber the intruder would see a stronger chaff signalcontribution than a true signal. However, if the out-coupling of thechaff signal is much larger than that of the true signal, then the chaffsignal can be stripped out, thereby exposing the true signal to theintruder further down the link. As a result, in an embodiment, theout-coupling coefficient for both chaff and true signal channels may benearly equal over the entire length of the cable.

With respect to attenuation, for α (dB/m) being the attenuation per unitlength of fiber for chaff and true signal channels, the followingconditions are possible, where α_(chaff) is the attenuation for thechaff signal and α_(signal) is the attenuation for the true signal.

If α_(chaff)>α_(signal), then it is possible that after a sufficientdistance traveled in the cable link, the chaff signal will besufficiently weaker than the legitimate signal and there will be noprotection of the legitimate data from an intruder tapping into thefiber cable.

If α_(chaff)<α_(signal), then as long as the signal attenuation isacceptable over the link, then adequate chaff protection is possibleagainst an intruder tapping the fiber cable.

If α_(chaff)=α_(signal), then the fiber can be secured along its entirelength.

The conclusion is that the signal attenuation must be sufficiently smallto ensure good signal quality over the link and that the chaffattenuation must be equal to or smaller than the signal attenuation toenable protection over the entire link.

With respect to crosstalk, for γ (dB/m) being the crosstalk per unitlength of fiber between chaff and true signal channels, the followingconditions are possible, where γ_(chaff) is the crosstalk of the chaffsignal and γ_(signal) is the crosstalk of the true signal.

If the crosstalk, γ, between chaff channel(s) and the signal channel issignificant then this can limit the data transfer along the legitimatesignal channel. It may also be easier to tap the fiber as both signaland chaff channels will contain signal information.

According to an embodiment the aggregate optical power of all theoptically modulated chaff sources within the optical frequency rangesubstantially occupied by an optically modulated transmit source ischosen such that the ratio of chaff power to transmit signal powercoupled out at a fiber bend anywhere along the link is smaller than therequired signal-to-noise ratio specified by the transmit signal'scorresponding receiver to achieve a desired or maximum error orerror-free performance at its output.

According to an embodiment the optical powers of at least one opticallymodulated chaff source within the optical frequency range substantiallyoccupied by an optically modulated transmit source is chosen such thatthe ratio of chaff power to transmit signal power coupled out at a fiberbend anywhere along the link is smaller than the requiredsignal-to-noise ratio specified by the transmit signal's correspondingreceiver to achieve error-free performance at its output.

According to an embodiment the aggregate optical power of all theoptically modulated chaff sources within the optical frequency rangesubstantially occupied by an optically modulated transmit source ischosen such that the chaff optical power coupled out at a fiber bendanywhere along the link is larger than that transmit signal's opticalpower coupled out at a fiber bend at the same point along the link.

According to an embodiment the optical power of at least one opticallymodulated chaff source within the optical frequency range substantiallyoccupied by an optically modulated transmit source is chosen such thatthe chaff optical power coupled out at a fiber bend anywhere along thelink is larger than that transmit signal's optical power coupled out ata fiber bend at the same point along the link.

If crosstalk is low between the signal and chaff channels, then thesecured fiber link system will be the most effective. Thus, crosstalkshould be minimized in the fiber.

FIG. 3 shows an example cross section of an optical fiber 300 showing arefractive index profile according to an embodiment. The fiber includeslayer n₁ 310, n₂ 320, n₃ 330, and n₄ 340. The relationship between therefractive index of the different layers is: n₁>n₂, n₃>n₂ and n₃>n₄.There is no specific relation between n₂ and n₄, and these two valuesmay be equal or unequal. In the most basic implementation, the signalmay be launched in the center core (n₁) 310 and the chaff signal can belaunched in the second “ring” core (n₃) 330. The other layers, n₂ 320and n₄ 340, act as cladding confining the true signals and chaff signalsto their respective “cores.” The fiber 300 has radial symmetry, whichmeans that it will behave the same when tapped by bending it in anydirection.

In an example implementation, the optical fiber 300 may be utilized in asecured fiber link system (e.g., the secured fiber link system 100,FIG. 1) in which a desired sequence of information is transported overthe optical fiber. The sequence of information includes a first“legitimate” or “true” signal (or set of signals) that is typicallyintended to be transferred between legitimate users at both ends of alink. Chaff signals, or subterfuge signals, are signals sent alongsidethe true signal to prevent intruders from tapping such legitimatesignals from a fiber cable. The optical fiber 300 can work in concertwith other measures for protecting transmitted data such as dataencryption, patrolling of data lines by guards, intrusion detectionmonitor sensors, and hardening of data lines by encasing them inconcrete or steel conduits.

Thus, in an embodiment, the resulting protection is the same regardlessof how an intruder applies their fiber bending mechanism. In anembodiment, the chaff and true signal channels can both be coupledselectively into the true signal and chaff signal channels of the fiber100 at a transmitter end (not shown) and selectively coupled out atanother end (e.g., a receiver end, not shown) of the link with anappropriate coupler. Such a coupler may include, but is not limited to,a lensed fiber based coupler, a tapered glass fiber coupler, and a freespace bulk optics coupler.

In a typical embodiment, the optical fiber medium should have thefollowing properties that both maximize the effectiveness of theanti-tapping capability of the system and do not inhibit the legitimateinformation transfer between system users: out-coupling efficiency ofthe true and chaff channels via bending, attenuation of the chaff andtrue signal channels, and cross-talk between the chaff and true signalchannels.

In an embodiment, the fiber 300 is designed to meet the criteriaoutlined herein above with respect to out-coupling, attenuation, andcross-talk, for tapping protection while also providing a transmissionchannel that is equivalent in capacity and performance as standardoptical transmission fibers. The fiber 100 combines the simplicity ofmanufacturing with the added capability of anti-tapping protection in asecure fiber link system. Specifically, in an embodiment, the fiber 100includes one “true” signal path and one chaff signal path. The fiber 100is structured using an assembly of concentric, circularly symmetricglass tubes and and/or glass rods. This allows to manufacture the fiber100 using conventional optical fiber draw technology.

In another embodiment, the core can be single mode, thereby maximizingthe bandwidth that the true signal channel can accommodate. The fiber300 requires the minimum number of transceivers at the terminals of thesecure link since it only requires one signal and one chaff channel toprovide tapping protection no matter in which direction the fiber isbent by an intruder. A number of commercially available fiber couplerscan be used to couple both chaff and true signal channels in to and outof the fiber 300. In an embodiment, the fiber 300 can operate in asystem that includes existing mechanisms for detecting intrusion ortampering with the fiber link.

In an embodiment, tapping the fiber 300 anywhere along the link willcapture energy from both the intended signal core and from the second(“ring”) core carrying chaff signal whereas the intended recipient ofthe signal information content (at the other end of the link) is notexposed to the chaff signal at the terminal equipment. The chaff signalchannels received at the terminal equipment can, however, be used tofacilitate intrusion detection and in fact, can be used in conjunctionwith intrusion detection.

In an embodiment, the chaff signals may be similar to the true signal insignal protocol/format, (sample) timing, and/or optical wavelength suchthat both the chaff signals and the true signal will pass through anydiscriminator, used by an intruder, based on any or all of those signalfeatures. Therefore, given that the chaff signal(s) have sufficientenergy, the intruder will be swamped by the chaff information which theycannot separate from the true signal information. Given that the chaffsignals are strong enough compared to the signal along the entire lengthof the cable, the intrusion defense will be strong everywhere along thecable without the need for additional protection such as cable armamentor posting sentries. In another embodiment, the signal and chaffchannels may also be utilized for intruder monitoring.

The untappable fiber system including the optical fiber disclosed hereinmay provide several advantages such as, but not limited to, protectionalong the entire length of the link without the need for expensiveguards or encasements making installation and security maintenance lessexpensive (especially on a long link), and increase in bandwidthavailable for transmitting data due to reduced dependence on dataencryption for security.

It should be noted that the various teachings herein are described withrespect to particular units of measurement merely for simplicitypurposes and without limitation on the disclosed embodiments. Forexample, the optical transmit source may contain a plurality ofwavelength-division multiplexed real signals and the optical chaffsource may contain a corresponding plurality of wavelength-divisionmultiplexed optically modulated chaff signals, or more or less than theplurality of wavelength-division multiplexed real signals. In anotherexample the optical transmit source is provided external to the entityproviding the optical chaff sources.

In other various embodiments, the optical transmit source is providedexternal to the entity providing the optical chaff sources. The entityproviding the optical chaff sources determines the optical frequencyrange occupied by one or more externally provided optical transmitsources. The optical frequency range of the optical chaff sources isadapted to the optical frequency range of the externally providedoptical transmit sources. The power of the optical chaff sources isadapted according to the optical power of the externally providedoptical transmit sources. The power of the one or more optical chaffsignals as well as the power of the one or more transmit signals aremeasured and the ratio of the measured chaff and transmit signal powersare compared to the ratio of launched chaff and transmit signal powers.An alarm is raised if the change of power ratios exceeds a predefinedthreshold.

It should be noted that the disclosed embodiments can be utilized inalternatively or in conjunction with existing or future fiber cablesand/or security systems for preventing tapping or other tampering withfiber cables without departing from the scope of the disclosure.

The various embodiments disclosed herein can be implemented as anycombination of hardware, firmware, and software. Moreover, the softwareis preferably implemented as an application program tangibly embodied ona program storage unit or computer readable medium. The applicationprogram may be uploaded to, and executed by, a machine comprising anysuitable architecture. Preferably, the machine is implemented on acomputer platform having hardware such as one or more central processingunits (“CPUs”), a memory, and input/output interfaces. The computerplatform may also include an operating system and microinstruction code.The various processes and functions described herein may be either partof the microinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU, whether or not suchcomputer or processor is explicitly shown. In addition, various otherperipheral units may be connected to the computer platform such as anadditional data storage unit and a printing unit. Furthermore, anon-transitory computer readable medium is any computer readable mediumexcept for a transitory propagating signal.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the disclosed embodiment and the concepts contributed by the inventorto furthering the art, and are to be construed as being withoutlimitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosed embodiments, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

What is claimed is:
 1. A secured fiber link system, comprising: aspatially multiplexing optical fiber; a transmit spatial multiplexerconfigured to couple each of a plurality of optical signals into arespective one of a plurality of distinct spatial paths of the spatiallymultiplexing optical fiber, each of the spatial paths being able tocarry an optical signal; at least one optically modulated transmitsource coupled to the transmit spatial multiplexer so that an opticallymodulated signal supplied as an output from the at least one opticallymodulated transmit source is coupled by the transmit spatial multiplexerinto one of the plurality of spatial paths of the spatially multiplexingoptical fiber; at least one optically modulated chaff source coupled tothe transmit spatial multiplexer so that each optically modulated chaffsignal supplied as an output from the at least one optically modulatedchaff source is coupled by the transmit spatial multiplexer into arespective one of the plurality of spatial paths of the spatiallymultiplexing optical fiber that are other than the spatial path to whichthe at least one optically modulated transmit source is coupled, whereinat least one of the at least one optically modulated chaff sourceoccupies substantially a same optical frequency range as at least one ofthe at least one optically modulated transmit source; and a receivespatial multiplexer configured to extract at least one spatial path fromthe spatially multiplexing fiber.
 2. The system of claim 1, wherein theparallel paths carried by the spatially multiplexing optical fiber arenominally uncoupled.
 3. The system of claim 1, wherein: the spatialmultiplexer comprises at least one of a lensed fiber based coupler, atapered glass fiber coupler, a polymer based coupler, or a free spacebulk optics coupler, each of the at least one the optically modulatedtransmit source is modulated using any one of on-off keying, phasemodulation or quadrature amplitude modulation, and the spatiallymultiplexing fiber is a multi-core fiber.
 4. The system of claim 1,configured by at least one of: at least one of the optically modulatedchaff sources employs substantially the same optical modulation formatas at least one of the at least one optically modulated transmit source,and at least one of the optically modulated chaff sources is temporallysynchronized with at least one of the at least one optically modulatedtransmit source.
 5. The system of claim 1, wherein at least one of theat least one optically modulated chaff sources contains a data stream.6. The system of claim 5, wherein the data stream is at least one of:generated at the chaff source in real time, pre-recorded and stored atthe chaff source, and generated at the chaff source using a quantumrandom number generator.
 7. The system of claim 1, wherein the receivespatial multiplexer is configured to extract at least one optical signalfrom the spatially multiplexing optical fiber, wherein the at least oneextracted optical signal is detected locally, and wherein the at leastone extracted optical signal is made available as a system output. 8.The system of claim 7, wherein a power of the at least one optical chaffsignal and a power of the at least one transmit signal are measured, andwherein a ratio of the measured optical chaff and transmit signal powersare compared at the receive spatial multiplexer to a ratio of at leastone optical chaff signal power and at least one transmit signal power asmeasured at the transmit spatial multiplexer.
 9. A method for securing afiber link, comprising: coupling a plurality of optical signals intorespective ones of a plurality of spatial paths of a spatiallymultiplexing optical fiber, wherein at least one of the plurality ofoptical signals is generated by an optically modulated transmit sourceto contain real data and wherein at least one of the plurality ofoptical signals is generated by at least one optically modulated chaffsource as an optically modulated chaff signal; wherein at least theoptically modulated chaff signal occupies substantially a same opticalfrequency range as at least the optical signal that is generated by theoptically modulated transmit source.
 10. The method of claim 9, furthercomprising: extracting by a receive spatial multiplexer at least onespatial path from the spatially multiplexing fiber.
 11. The method ofclaim 9, wherein: at least one of the optically modulated chaff sourcesemploys substantially the same optical modulation format as theoptically modulated transmit source, and at least one of the opticallymodulated chaff sources is temporally synchronized with at least theoptically modulated transmit source.
 12. The method of claim 9, whereinat least one of the at least one optically modulated chaff sourcescontains a data stream wherein the data stream is at least one of:generated at the chaff source in real time, pre-recorded and stored atthe chaff source, and generated at the chaff source using a quantumrandom number generator.
 13. The method of claim 9, wherein a ratio ofchaff and transmit signal powers measured at a receiver are compared toa ratio of chaff power and transmit signal power measured substantiallyat the location at which the plurality of optical signals are coupledinto the spatially multiplexing fiber.
 14. The method of claim 13,further comprising: triggering an alarm when the comparison indicatesthat a change of the power ratios has exceeded a predefined threshold.15. A fiber terminal, comprising: an optical coupler for interfacingbetween at least one individual true signal channel and at least onechaff signal channel and an optical cable link; a chaff clocksynchronizer configured for clock recovery and synchronization; a chaffsignal generator configured to drive the at least one chaff channel insynchronization with the at least one true signal channel; and at leastone transceiver configured to convert between electrical input/outputsignals and optical signals that are transmitted over the optical cablelink.